The present invention relates to a vacuum furnace for the heat treatment of metallic workpieces having a cylindrical pressure shell in which are disposed a charge chamber surrounded by axially aligned heating conductors and provided with a thermal insulation and a gas-cooling device, with which a coolant gas can be passed through nozzles through the charge chamber and through a heat exchanger. Such vacuum furnaces are particularly used for the hardening of all kinds of tools and structural parts of many different steel grades. In some cases they can also be used for other heat treatments, e.g., annealing and soldering.
Vacuum furnaces in general are described in West German Patents Nos. 2,839,807 and 2,844,843. They include as essential components a cylindrical pressure shell in which is located a charge chamber bounded by thermal insulation walls and heated with heating elements, and a gas-cooling device. The tools and structural parts are heated under vacuum in the charge chamber to the austenitizing temperature and, for quenching, a cooled inert gas is circulated in the furnace under pressure. In the process, the coolant gas flows at high velocity onto the hot charge, removes heat energy therefrom and is passed through a heat exchanger, where it is cooled and returned to the charge chamber. According to West German Patent No. 2,839,807, the coolant gas is injected into the charge chamber through nozzles, which are attached to separate axially aligned gas-inlet conduits. One disadvantage of this construction is the high material and fabrication costs for the gas-inlet conduits in the furnace. Conduits and nozzles must consist of refractory material. The fans used in West German Patent No. 2,844,843 have the drawback that the coolant gas flows to a considerable extent only along the hot charge surface and does not penetrate into the charge interior.
West German Laid-open Application 1,919,493 teaches how to accelerate the heating of the charge in the temperature range between room temperature and approximately 750.degree. C. by circulating an inert gas into the furnace by means of a fan and thereby to generate air in addition to the radiation. Here, too, however, there is no optimum heat transfer between heating conductors and charge.